Method for producing a cathode steel bar with copper insert, and method for removing a copper insert from a used cathode bar

ABSTRACT

The present invention relates to a method for producing a cathode steel bar with copper insert for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process. The present invention further relates to a method of removing a copper insert from a cathode bar used in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, and reusing the copper from old copper inserts in production of new cathode bars with copper insert.

TECHNICAL FIELD

The invention relates to the technical field of electrolysis in moltensalts for making aluminium using the Hall-Héroult process. Moreprecisely, the present invention relates to improved cathode steel barswith copper insert, a method for producing such cathode steel barspossibly from used cathode steel bars, and a method for removing copperinserts from used cathode steel bars.

BACKGROUND ART

Aluminium is commonly produced by electrolysis of alumina (aluminiumoxide) dissolved in a molten cryolite bath. A Hall-Héroult typeelectrolytic cell for such production comprises a steel shell (potshell) with a lining of refractory material, where the bottom of thecell is a carbon cathode having several current collectors embeddedtherein, and several anode blocks that are partly submerged in theelectrolyte bath, and arranged at a distance above the cathode. Inindustrial production of aluminium the cells of the Hall-Héroult typeare connected electrically in series, and the solution of alumina inmolten cryolite is brought to a temperature up to about 980° C. by theheating effect of the current traversing through the cell. The thusformed aluminium metal accumulates in the cell bottom, on the cathodesurface, and is regularly tapped from the cell.

In more detail, the cathode consists of a carbon liner with severalcathode steel bars to conduct electric current out of the cell. Thecathode steel often has copper inserts at least along parts of thelength of the cathode steel, in order to improve the distribution ofcurrent along the cathode, see e.g. NO 343609, WO 01/63014, WO 01/27353.

The lifetime of a cathode in an electrolysis cell may be a few yearsbefore relining of the cell is needed. Used cathode steel with copperinserts usually has a low value for recycling, and must be replaced bynew ones in a lined cell.

There are various ways to produce cathode steel with a copper insert.One method is to drill a longitudinal hole in the steel bar with adiameter corresponding to a copper rod to be inserted. The steel bar isheated and crimped around the copper rod. This method requires highprecision as the clearance between the hole in the steel and the copperrod must be small, approx. 0.5 mm, to ensure sufficiently good contactbetween the steel and the copper insert, which is important to obtaingood conductivity. The said method also has limitations in terms of thedesign of the cross section of the copper insert.

Therefore, there is a desire for an improved method of producing cathodesteel bars with copper insert of good quality, e.g. in terms ofconductivity, and without pores or suction in the copper inserts. Thus,it is an objective of the present invention to mitigate, alleviate oreliminate one or more of the disadvantages of today's solutions in thistechnical field.

SUMMARY OF INVENTION

In a first aspect, the present invention relates to a method ofproducing a cathode steel bar with copper insert for use in anelectrolytic cell for the electrolytic production of aluminium using theHall-Héroult process, comprising:

-   -   i) providing a steel portion of the cathode steel bar equipped        with a cavity for the copper insert and a sleeve on the upper        part,    -   providing one or more solid copper item(s) sized to be inserted        into the cavity of the steel portion,    -   entering the copper item(s) into the steel portion through the        sleeve, positioning at least one inductor in close proximity to        at least parts of the outside of the steel portion,    -   supplying electric energy to the inductor, causing induction        heating of the cathode steel bar to a temperature sufficiently        high and for a time sufficiently long to molten at least the        outer part of the copper item(s),    -   cooling, and solidifying the molten copper;    -   or    -   ii) providing a steel portion of the cathode steel bar equipped        with a cavity for the copper insert and a sleeve on the upper        part,    -   positioning at least one inductor in close proximity to at least        parts of the outside of the steel portion,    -   supplying electric energy to the inductor, causing induction        preheating of the cathode steel bar,    -   providing molten copper,    -   pouring the molten copper into the cavity of the steel portion        through the sleeve, cooling, and solidifying the molten copper.

The method of the present invention may be a circular process wherecopper insert of an used cathode bar is the source of the copper insertof the cathode bar to be prepared. That is, the present inventionprovides a method enabling recyclability of used cathode bars withcopper insert. This is advantageous in terms of flexibility, productiontime and cathode performance, as well as it is environmental friendly.

In a second aspect the present invention relates to a cathode steel barwith copper insert for use in an electrolytic cell for the electrolyticproduction of aluminium using the Hall-Héroult process, obtained by theprocess above.

The present invention provides cathode bars with improved performance interms of conductivity.

In a third aspect the present invention relates to a method of removinga copper insert from an used cathode bar used in an electrolytic cellfor the electrolytic production of aluminium using the Hall-Héroultprocess, comprising:

-   -   providing an used cathode bar,    -   positioning an inductor in close proximity to at least parts of        the cathode bar, supplying electric energy to the inductor,        causing induction heating of the cathode bar to a temperature        above the melting temperature of copper, forming molten copper,        pouring the molten copper into a holding furnace; or    -   casting the molten copper into suitable items.

Hence, it is to be understood that the herein disclosed invention is notlimited to the particular component parts of the device described orsteps of the methods described since such device and method may vary. Itis also to be understood that the terminology used herein is for purposeof describing particular embodiments only, and is not intended to belimiting. It should be noted that, as used in the specification and theappended claim, the articles “a”, “an”, “the”, and “said” are intendedto mean that there are one or more of the elements unless the contextexplicitly dictates otherwise. Thus, for example, reference to “a unit”or “the unit” may include several devices, and the like. Furthermore,the words “comprising”, “including”, “containing” and similar wordingsdoes not exclude other elements or steps.

The terms “cathode bar(s)”, “cathode steel bar(s)”, “steel bar(s)”,“current collector(s)”, “cathode collector bar(s)”, “collector bar(s)”and “current collecting bar(s)”, may be used interchangeably in thepresent disclosure, and should be understood to denote the same entity,unless other stated.

The term «copper» as used herein includes pure copper and copper alloys.

The term “cavity” as used herein is to be understood as a confined spacethat may be open in one or more ends. A longitudinal hole in a cathodebar, open in one or both ends, is included in this term.

The terms “used cathode bar(s)” and “old cathode bar(s)” or anyvariations of the term “cathode bar(s)” as defined above starting withthe word “used” or “old”, may be used interchangeably in the presentdisclosure, and should be understood to denote the same entity, unlessother stated.

The terms “collar” and “sleeve” may be used interchangeably and has thefunction as feeder in the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 : Illustrates a cathode bar according to the invention producedby inserting a solid copper item into the steel portion.

FIG. 2 : Illustrates a cathode bar according to the invention producedby pouring molten copper into the steel portion (top filling).

FIG. 3 : Illustrates a cathode bar according to the invention producedby filling molten copper into the steel portion from the bottom/lowerpart.

FIG. 4 : Illustrates a cathode bar according to the invention producedby filling molten copper into the steel portion from the bottom/lowerpart and using counter gravity casting.

FIG. 5 : Illustrates a cathode bar according to the invention producedby filling molten copper into the steel portion from the bottom.

FIG. 6 : Illustrates a further cathode bar according to the inventionproduced by filling molten copper into the steel portion from thebottom.

FIG. 7 : Illustrates an “over the lip” filling device for filling moltenin production of a cathode bar with copper insert.

FIG. 8 : Illustrates a bottom tapping device for filling molten copperin production of a cathode bar with copper insert.

FIG. 9 : Illustrates removing of a copper insert from an old cathode baraccording to the invention.

FIG. 10 : Photo of a section of a cathode bar with copper insertaccording to the invention.

The FIGS. 1-6 and 9 illustrates various embodiments of correspondingcathode bars. Thus, the structural parts of the cathode bars have thesame reference numbers.

DETAILED DESCRIPTION

In the following, the present method for producing a cathode steel barwith copper insert for use in an electrolytic cell for the electrolyticproduction of aluminium using the Hall-Héroult process, will bedescribed and explained by way of examples and with reference to theaccompanying drawings in which the same reference numbers refer to thesame or technically equivalent elements, unless otherwise stated.

FIG. 1 illustrates one embodiment of the present invention, showing across-section of a cathode bar comprising a steel portion 2 equippedwith a cavity for insertion of solid copper, and a collar or sleeve 4 ofa metal material such as steel or any other suitable refractory materialat the upper part acting as a feeder. The here illustrated cathode barcomprises a copper insert 1 designed as a solid rod/bar. A closingdevice 5 of a suitable material to prevent heat loss such as steel,titan etc., may be placed at the upper end of the cathode bar to ensureproper seal of the copper insert 1 within the steel portion 2.

It should be understood that the solid copper which is the startingmaterial of the copper insert can have several designs, e.g. a circularrod, or a rod of any geometry, or ingots or bars of any design. The onlyrequirement of the solid copper item is that it is sized to be insertedinto the cavity of the steel portion. For instance, the solid copper maybe a rod with a smaller diameter than the cavity in the steel portion ofthe cathode bar. Another alternative is to use several copper items suchas ingots and bars.

In an embodiment the method comprises entering one or more solid copperitem(s) 1 is/are into the steel portion 2 through the sleeve 4, andheating the steel portion 2, preferably using induction heating, untilat least the outer part of the solid copper items inserted therein ismolten. It may also be heated until the solid copper is fully molten.

In induction heating a work-piece is heated by eddy currents induced inthe work-piece. An induction heating power supply converts alternatingcurrent (AC) line power to a higher frequency AC and delivers the higherfrequency AC to an inductor wherein an electromagnetic field is createdwithin the coil of the inductor. Eddy currents will be induced in anelectrically conductive work-piece placed in the electromagnetic field,generating heat in the work-piece. The inductor is commonly water cooledcopper conductors made of e.g. copper tubes, profiles, plates ormachined copper parts. The design of the inductor influences inter aliaenergy transfer to work-piece, heating rate and heating efficiency.Inductors may be helix formed or prepared from plates geometry, and thenumber of windings may vary. The present invention is not limited to anyspecific design or type of inductor. Furthermore, one or more inductors,which can be controlled separately or as a whole, may be used. Forinstance, the inductor may appear as being divided.

At least one inductor 3 is positioned in close proximity to at leastparts of the outside of the steel portion 2. Preferably, the inductor 3is positioned such that it encircles at least partially the part of thesteel portion 3 comprising the inserted copper item(s) 1. The inductor 3is connected to a power supply (not shown in the drawings) whichconverts AC line power to an AC having a frequency of between 1 kHz and50 kHz. The converted AC is delivered to the inductor 3, causinginduction heating of at least a part of the steel portion 2 comprisingcopper item(s), encircled by the inductor 3. The steel induction heatedpart of the steel portion 2 is heated to a temperature above the meltingtemperature of copper or copper alloy. The induction heating iscontinued at least until the outer part of the solid copper items ismolten, forming a partly or fully molten pool of copper within in thecavity of the steel portion 2.

In one embodiment, the induction heating is continued after the solidcopper item(s) have become molten in order to increase performance ofthe cathode bar. The induction heating may be held for a period of timevarying from 10 seconds to 12 hours, preferably a period of 1 minute to1 hour.

After the induction heating, the thus heated part of the steel portion 2containing the partly or fully molten copper is cooled. Preferably, thecooling is controlled such that the lower part of the heated area iscooled first leading to solidification of the molten inner portion ofcopper, while the upper part is kept molten. The cooling is preferablycarried out directionally such that the copper in the upper part of thesteel portion is lastly solidified. Directional cooling may be obtainedby upward moving of the inductor length wise along the cathode bar.Optionally, the bottom part of the steel portion 2 may be subjected toadditional cooling in order to initiate the directional cooling. By suchcontrolled cooling, piping and formation of shrink holes is reduced andcontrolled. A copper insert of good quality in that it is virtuallypore-free and with no suctions is ensured in the cathode bar.

FIG. 2 illustrates another embodiment of the present invention, showinga cross-section of a cathode bar comprising a steel portion 2 equippedwith a cavity for filling of molten copper 6, and a collar or sleeve ofa metal material such as steel or any other suitable refractory materialat the upper part acting as a feeder.

In this embodiment, the method comprises preheating of the steel portion2 of the cathode by residual heat from previously heated cathode barsand/or by utilizing induction heating.

At least one inductor 3 is positioned in close proximity to at leastparts of the outside of the steel portion 2. The properties and designof the inductor to be used in this embodiment correspond to theaforementioned description of inductor.

Preferably, the inductor 3 is positioned such that it encircles at leastpartially the part of the steel portion 3 comprising the cavity forfilling molten copper. The inductor 3 is connected to a power supply(not shown in the drawings) which converts AC line power to an AC havinga frequency of between 1 kHz and 50 kHz. The converted AC is deliveredto the inductor 3, causing induction heating of at least a part of thesteel portion 2 encircled by the inductor 3. The steel induction heatedpart of the steel portion 2 is heated to a temperature above the meltingtemperature of copper or copper alloy.

Molten copper is provided, e.g. from used cathode bars or any othersource, and filled into the steel portion 2 of the cathode bar, eitherfilling from top or bottom with a suitable casting method.

In one embodiment, the induction heating of the steel portion 2 iscontinued after filling of molten copper into the cavity of the steelportion 2 in order to increase performance of the cathode bar. Theinduction heating of may be held for a period of time varying from 10seconds to 12 hours, preferably a period of 1 minute to 1 hour.

After the induction heating, the thus heated part of the steel portion 2containing the partly or fully molten copper is cooled. Preferably, thecooling is controlled such that the lower part of the heated area iscooled first leading to solidification of the molten inner portion ofcopper, while the upper part is kept molten. The cooling is preferablycarried out directionally such that the copper in the upper part of thesteel portion is lastly solidified. Directional cooling may be obtainedupward moving of the inductor length wise along the cathode bar.Optionally, the bottom part of the steel portion 2 may be subjected toadditional cooling in order to initiate the directional cooling. By suchcontrolled cooling, piping and formation of shrink holes is reduced andcontrolled. A copper insert of good quality in that it is virtuallypore-free and with no suctions is ensured in the cathode bar.

FIG. 3 illustrates further embodiment of the present invention wheremolten copper is filled into the steel portion from the bottom orthrough an opening in the steel portion 2 placed anywhere along its sidebelow the sleeve 4 at the top, i.e. through a permanent steel mold withgating. FIG. 3 is showing a cross-section of a cathode bar comprisingsaid a permanent steel mold with gating/a steel portion 2 equipped witha cavity for filling of molten copper, and a collar or sleeve 4 of ametal material such as steel or any other suitable refractory materialat the upper part. A downsprue 7 for filling of molten copper isconnected to the steel portion 2 so that the molten copper is filledinto the cavity of the steel portion. The connection point is shown atthe lower part of the steel portion, but may be anywhere along the sideof the steel portion 2. Only one steel portion 2 is shown, but typicallya number of steel portions 2/permanent steel molds with gating areconnected to the downsprue 7 for simultaneous filling of molten copper.At least one inductor 3 is positioned in close proximity to at leastparts of the outside of the steel portion 2 in a similar way asexplained with regard to FIG. 2 above. Molten copper is filled at adesired level in the cavity of the steel portion 2 and the sleeve 4 actsas a prolongation of the cavity to protect against overfilling and willsecure good quality copper. The collar/sleeve 4 may also be a designedas a separate part connected to the steel portion 2. Heating and coolingare performed as discussed for the embodiment of FIG. 2 . The inductor 3will direct the solidification towards the collar/sleeve 4. Duringcooling the copper level decreases. Solidified copper on the edge of thesleeve 4 may be removed by means of drilling, machining or any othersuitable method, and a closing device 5 (not shown) may be introduced.

During filling of the molten copper, the steel portion 2 of the cathodebar is preferably kept at an angle to reduce pressure height and toimprove the filling process by avoiding turbulence and/or oxideformation. After the filling the steel portion 2 with molten copper, thesteel portion 2 may be tilted to a vertical position or any otherposition. This embodiment is illustrated in FIG. 4 showing a cathode baraccording to the invention produced by filling molten copper 6 into thesteel portion 2 at the lower end and equipped with an inductor 3, byusing counter gravity casting.

FIG. 5 illustrates further embodiment of the present invention wheremolten copper is filled into the steel portion from the bottom of thesteel portion 2. FIG. 5 is showing a cross-section of a cathode barcomprising said a steel portion 2 equipped with a cavity for filling ofmolten copper, and a collar or sleeve 4 of a metal material such assteel or any other suitable refractory material at the upper part. Adownsprue 7 for filling of molten copper is connected to the steelportion 2 so that the molten copper is filled into the cavity of thesteel portion from its bottom/underside. Only one steel portion 2 isshown, but typically a number of steel portions 2 are connected to thedownsprue 7 for simultaneous filling of molten copper. At least oneinductor 3 is positioned in close proximity to at least parts of theoutside of the steel portion 2 in a similar way as explained with regardto FIG. 2 above. Molten copper is filled at a desired level in thecavity of the steel portion 2 and the sleeve 4 acts as a protectionagainst overfilling and will secure good quality copper. The sleeve 4shown in FIG. 5 is placed at the top of the steel portion 2, but anylocation further down on the side of the steel portion 2 is within thescope of the invention. Heating and cooling are performed as discussedfor the embodiment of FIG. 2 . The inductor 3 directs the solidificationtowards the sleeve 4. During cooling the copper level decreases.Solidified copper on the edge of the sleeve 4 may be removed by means ofdrilling, machining or any other suitable method, and a closing device 5(not shown) may be introduced.

FIG. 6 illustrates still a further embodiment of the present inventionwhere molten 2 copper is filled into the steel portion from the bottomof the steel portion 2. FIG. 6 is showing a cross-section of a cathodebar comprising said a steel portion 2 equipped with a cavity for fillingof molten copper, and a collar or sleeve 4 of a metal material such assteel or any other suitable refractory material at the upper part. Areservoir of molten copper equipped with a feeder P for addition ofmolten copper to the reservoir and a supply pipe 8 connected to thesteel portion 2, is arranged below the steel portion 2. Molten copper isfilled into the cavity of the steel portion from its bottom/undersidethrough the supply pipe 8. At least one inductor 3 is positioned inclose proximity to at least parts of the outside of the steel portion 2in a similar way as explained with regard to FIG. 2 above. Molten copperis filled at a desired level in the cavity of the steel portion 2 andthe sleeve 4 acts as a protection against overfilling and secure goodquality copper. The sleeve 4 shown in FIG. 6 is placed at the upper partof the steel portion 2, but may be located further down on the side ofthe steel portion. Heating and cooling are performed as discussed forthe embodiment of FIG. 2 . The inductor 3 directs the solidificationtowards the sleeve 4. During cooling the copper level decreases.Solidified copper on the edge of the sleeve 4 may be removed by means ofdrilling, machining or any other suitable method, and a closing device 5(not shown) may be introduced.

FIG. 7 illustrates an “over the lip” filling device and FIG. 8 a bottomtapping device for filling molten copper in production of a cathode barwith copper insert. Both devices are commercial available and within theknowledge of the skilled person. “Over the lip” filling and bottomtapping are useful filling methods for carrying out the embodiments ofthe invention related to filling molten copper when producing thecathode bars with copper insert such as illustrated in FIGS. 2-6 .

FIG. 9 illustrates removing of a copper insert 1 from a used cathodebar. A cross-section of a cathode bar comprising a steel portion 2 and acopper insert 1 is shown illustrating the embodiment of the presentinvention where copper is melted out for optional re-use in productionof a new cathode steel bar as described in the embodiment above.

In this embodiment, a used cathode steel bar is provided and preparedfor copper melt out, typically through a cutting process or similar togain direct access to the copper insert of the cathode bar.

The cathode bar is preferably preheated by rest heat from previouslyheated cathode bars and subsequently heated by utilizing inductionheating to at least the melting point of copper or copper alloy.

At least one inductor 3 is positioned in close proximity to at leastparts of the outside of the steel portion 2. The properties and designof the inductor to be used in this embodiment correspond to theaforementioned description of inductor. Preferably, the inductor 3 ispositioned such that it encircles at least partially the part of thecathode bar comprising the copper insert. The inductor 3 is connected toa power supply as described above and causes induction heating of atleast a part of the cathode bar encircled by the inductor 3. Whenreaching the melting point of the copper, molten copper is formed andpoured out of the steel portion 2 of the cathode bar into a holdingfurnace, or casted directly into copper ingots or similar solid copperitems.

In the embodiment where molten copper is poured into a holding furnace,it may be re-used directly in molten state for production of new cathodebars as described above in relation to FIGS. 2-8 .

In the embodiment where the copper is casted into solid copper items,they may be re-used for production of new cathode bars as describedabove in relation to FIG. 1 , or FIGS. 2-6 by re-melting of the solidcopper items.

Refining or “dilution” of the recycled copper to desired quality mightbe required. Here “dilution” means addition of more pure copper.

FIG. 10 shows a section of a cathode bar with copper insert prepared bythe method of the invention. It is clear from the photo of FIG. 10 thata unique intermetallic connection between steel and copper has beenachieved. Furthermore, no pores or suction can be seen in the copperinsert. Thus, it is shown that the method of the present inventionenables production of cathode bars with copper insert where the metals(steel and copper) are homogeneously joined. That is, the quality of thecathodes obtained are very good and their performance being improved interms of conductivity. Such cathode bars are not previously known.

Having described preferred embodiments of the invention it will beapparent to those skilled in the art that other embodimentsincorporating the concepts may be used. These and other examples of theinvention illustrated above are intended by way of example only and theactual scope of the invention is to be determined from the claims.

1. A method for producing a cathode steel bar with copper insert for usein an electrolytic cell for the electrolytic production of aluminiumusing the Hall-Héroult process, comprising: i) providing a steel portionof the cathode steel bar equipped with a cavity for the copper insertand a sleeve on the upper part, providing one or more solid copperitem(s) sized to be inserted into the cavity of the steel portion,entering the copper item(s) into the steel portion through the sleeve,positioning at least one inductor in close proximity to at least partsof the outside of the steel portion, supplying electric energy to theinductor, causing induction heating of the cathode steel bar to atemperature sufficiently high and for a time sufficiently long to moltenat least the outer part of the copper item(s), cooling, and solidifyingthe molten copper; or ii) providing a steel portion of the cathode steelbar equipped with a cavity for the copper insert and a sleeve on theupper part, positioning at least one inductor in close proximity to atleast parts of the outside of the steel portion, supplying electricenergy to the inductor, causing induction preheating of the cathodesteel bar, providing molten copper, pouring the molten copper into thecavity of the steel portion through the sleeve, cooling, and solidifyingthe molten copper, wherein the cooling is carried out directionally, ina direction from the lower part of the cathode bar towards its upperpart.
 2. The method, according to claim 1, comprising supplying electricenergy with a frequency of between 1 kHz to 50 kHz from a power unit tothe inductor.
 3. The method, according to claim 1, wherein the heatingand preheating temperature is above the melting temperature of copper orcopper alloy.
 4. The method, according to claim 1, wherein the heatingand preheating be held for a period of 10 seconds to 12 hours. 5.(canceled)
 6. The method, according to claim 1, wherein the directionalcooling is obtained by upward moving of the inductor length wise alongthe cathode bar.
 7. The method, according to claim 1, wherein the methodis circular in that the solid copper item(s) or molten copper isprovided from used cathode bars.
 8. The method, according to claim 1,wherein the solid copper item(s) is provided from used cathode bars. 9.The method, according to claim 1, wherein pouring of the molten copperinto the cavity of the steel portion is obtained by top filling.
 10. Themethod, according to claim 1, wherein filling of the molten copper intothe cavity of the steel portion is carried out by top or bottom filling.11. The method, according to claim 9, wherein the steel portion of thecopper bar is kept at an angle during filling of molten copper.
 12. Themethod, according to claim 1, wherein the molten copper is provided fromused cathode bars.
 13. A cathode steel bar with copper insert for use inan electrolytic cell for the electrolytic production of aluminium usingthe Hall-Héroult process, obtained by the process according to claim 1.14. A method of removing a copper insert from a cathode bar used in anelectrolytic cell for the electrolytic production of aluminium using theHall-Héroult process, comprising: providing an used cathode bar,positioning an inductor in close proximity to at least parts of thecathode bar, supplying electric energy to the inductor, causinginduction heating of the cathode bar to a temperature above the meltingtemperature of copper, forming molten copper, pouring the molten copperinto a holding furnace; or casting the molten copper into suitableitems.
 15. The method, according to claim 14, comprising supplyingelectric energy with a frequency of between 1 kHz to 50 kHz from a powerunit to the inductor.
 16. The method, according to claim 4, wherein theheating and preheating be held for a period of 1 minute to 1 hour. 17.The method, according to claim 10, wherein the steel portion of thecopper bar is kept at an angle during filling of molten copper.